H-NMR studies of water interactions in sitka spruce and western hemlock: Moisture content determination and second moments

The purpose of this study was to examine the influence of the moisture level on the cell-wall material in wood using pulsed proton nuclear magnetic resonance. The wood species used were western hemlock (Tsuga heterophylla (Raf.) Sarg.) and sitka spruce (Picea sitchensis (Bong.) Carr.), distinguishing between heartwood and sapwood regions. The moisture contents of the specimens were below the fibre saturation point and they were conditioned to equilibrium moisture contents based on initial desorption, adsorption and secondary desorption processes. From the FID experiments, the NMR-based moisture contents and the solid-wood lineshape second moments were determined. Average relative proton-spin densities, which were needed to calculate the NMR-based moisture contents, were determined from known moisture contents and they were: hemlock sapwood: 0.616; hemlock heartwood: 0.537; spruce sapwood: 0.679; and, spruce heartwood: 0.446. The average RSD value, considering both heartwood and sapwood, for western hemlock species was 0.577 and for sitka spruce was 0.563; these are close to published RSD values for other species. The condition as to how the equilibrium moisture content was attained did not influence the second moment for hemlock; however, for spruce sapwood, the second moments were sorption dependent. The hemlock M(2) decreased from about 5.1 x 10(9) s(-2) at low M(NMR) to 4.5 x 10(9) s(-2) (heartwood) and 4.3 x 10(9) s(-2) (sapwood) at higher M(NMR). The adsorption and secondary desorption M(2) for the spruce sapwood region decreased from about 5.0 x 10(9) s(-2) at low M(NMR) to about 4.1 x 10(9) s(-2) near the M(F), whereas M(2) for the spruce heartwood decreased from about 4.3 x 10(9) s(-2) at low M(NMR) to about 3.5 x 10(9) s(-2) near M(F). Extractives may have a key role in obtaining the RSD and second moments.